1. Field of the Invention
The invention relates to nanoporous dielectric films and to a process for their manufacture. Such films are useful in the production of integrated circuits.
2. Description of the Prior Art
In the production of integrated circuits, the problems of interconnect RC delay, power consumption and crosstalk become more significant as feature sizes approach 0.25 .mu.m and below. The use of low dielectric constant (K) materials for interlevel dielectric and intermetal dielectric applications partially mitigate these problems. However, each of the material candidates which are under consideration by the industry, having dielectric constants significantly lower than the currently employed dense silica, suffer from disadvantages. Most low dielectric constant materials developments use spin-on-glasses and fluorinated plasma chemical vapor disposition SiO.sub.2 with K of &gt;3. Some organic and inorganic polymers have dielectric constants in the range of about 2.2 to 3.5, however, these have the problems of low thermal stability, poor mechanical properties including low glass transition temperature, sample outgassing, and long term reliability questions.
Another approach has been to employ nanoporous silica which can have dielectric constants in the range of about 1 to 3. Porous silica is attractive because it employs similar precursors, e.g. tetraethoxysilane (TEOS) as is presently used for spun-on glass (SOG's) and CVD SiO.sub.2, and due to the ability to carefully control pore size and pore distribution. In addition to having low dielectric constants, nanoporous silica offers other advantages for microelectronics including thermal stability up to 900.degree. C.; small pore size (&lt;&lt;microelectronics features), use of materials, namely silica and its precursors, that are widely used in the semiconductor industry; the ability to tune dielectric constant over a wide range; and deposition using similar tools as employed for conventional spun-on glass processing. EP patent application EP 0 775 669 A2, which is incorporated herein by reference, shows a one method for producing a nanoporous silica film with uniform density throughout the film thickness.
Higher porosity materials not only leads to a lower dielectric constant than dense materials, but such also allow additional components and processing steps to be introduced. Materials issues include, the need for having all pores significantly smaller than circuit feature sizes; the strength decrease associated with porosity; and the role of surface chemistry on dielectric constant, loss and environmental stability. Density, or its inverse, porosity, is the key nanoporous silica parameter controlling property of importance for dielectrics. Properties may be varied over a continuous spectrum from the extremes of an air gap at a porosity of 100% to dense silica with a porosity of 0%. As density increases, dielectric constant and mechanical strength increase but the pore size decreases.
Nanoporous silica films are fabricated by using a mixture of a solvent and a silica precursor which is deposited on a wafer by conventional methods such as spin-coating, dip-coating, etc. The precursor must polymerize after deposition and be sufficiently strong such that it does not shrink during drying. Film thickness and density/dielectric constant can be controlled independently by using a mixture of two solvents with different volatility. The solvent evaporates during and immediately after precursor deposition. The silica precursor, typically a partially hydrolyzed and condensed product of TEOS, is polymerized by chemical and/or thermal means until it forms a gel layer. Using this approach, a nanoporous silica film is obtained with uniform density throughout the film thickness.
Normally in sol-gel processing to produce porous silica, a liquid catalyst such as an acid or base is added to the silica precursor/solvent mixture in order to initiate polymerization. This catalyst addition is often accompanied by the addition of water which is a reactant in the silane hydrolysis and condensation reactions which result in polymerization. For semiconductors processing, the requirement of premixing the catalyst with the precursor poses severe problems because relatively small quantities of catalyst and precursor, less than 5 milliliters total, must be very accurately measured and mixed since deposition is conducted one wafer at a time. The viscosity of the fluids start to change after catalyst addition which requires that deposition, which is a strong function of viscosity to achieve the same film thickness, must be carefully timed from catalyst addition to deposition. A catalyst/precursor solution which is not deposited at the correct time cannot be reused, leading to excessive waste.
In order to avoid these problems, a deposited precursor film is sequentially contacted with a water vapor and then a base vapor in either order to initiate polymerization. Previous investigators have contacted a carrier gas such as nitrogen with a base-water mixture such as ammonium hydroxide and passed the gas/vapor mixture by the wafer covered with the precursor film. Although avoiding the liquid-liquid mixing and deposition timing problems outlined above, this approach has the problems of maintaining constant ammonia and water partial pressures in the carrier stream; measuring the actual ammonia and water vapor concentrations in the carrier gas stream; relatively high flow rates are required to minimize reaction times; and disposal of large quantities of carrier gas containing ammonia. Another suggested approach has been to place the wafer with the precursor film in a sealed chamber and inject a base/water solution such as ammonium hydroxide into the chamber. This approach avoids some of the problems outlined above, but still does not allow measurement of the base and water concentration in the atmosphere and requires long reaction times because of gas phase mixing constraints in the atmosphere above the wafer.
The present invention solves these problems by conducting a series of processing steps which enable the production of nanoporous silica thin films with minimum process time, minimum base catalyst use, and with greater film uniformity of thickness and refractive index.